General Aviation Aircraft Manufacturing Process

 

Over the decades, certified and experimental general aviation has increasingly adopted composite materials, with composite manufacturing further driven by material and process innovations and the evolving AAM market.

 

 

The DarkAero1 is a long-range, high-speed, two-passenger, experimental, all-composite aircraft. The entire fuselage is made primarily of carbon fiber/epoxy and weighs about 200 pounds.

 

General aviation is a broad term that includes all civil aviation that is not large-scale, scheduled cargo and passenger operations. This means anything from ultralights to multi-engine turboprop and turbofan jets. For the purposes of this article, we will focus primarily on piston-powered fixed-wing aircraft that are typically used for business or pleasure travel. In the United States, there are about 175,000 aircraft flying to about 5,000 public airports, of which only about 10% have scheduled commercial flights.

 

Certified and experimental general aviation companies have been using fiber-reinforced composites for more than 60 years. In the late 1950s, Piper Aircraft (Vero Beach, Florida, USA) built an all-fiberglass prototype, the PA-29 Papoose. In the 1960s, fiberglass began to be widely used by glider manufacturers, who were always looking to reduce weight and improve lift-to-drag ratios.

 

After an eight-year certification process, in 1969 the Windecker Eagle I became the first all-composite powered aircraft to be certified by the Federal Aviation Administration (FAA) for its non-woven fiberglass “Fibaloy” and foam construction.

 

Composite experimental (home-built or kit) general aviation aircraft really took off in the early 1970s when Burt Rutan’s VariEze (and derivatives such as the Long EZ) became very popular. One of Rutan’s innovations was the use of sandwich foam as a “tool”, enabling “toolless” manufacturing of composites.

 

General aviation began using carbon fiber composites in the 1990s and slowly emerged from the recession that began in the 1980s. Major general aviation manufacturers such as Cirrus and Diamond introduced lines of single-engine certification aircraft that remain popular. Experimental aircraft soon moved to increasing carbon fiber content, and the new Light Sport Aircraft (LSA) category brought more opportunities for composites.

 

The three main categories of general aviation fixed-wing aircraft (certification, experimental, and LSA) all present varying degrees of challenges for composites.

 

Certified aircraft are government-approved (approved by the FAA) and require years of development and testing to prove their design and manufacturing process. Experimental aircraft are mostly built by individuals from plans or kits, and while there is some government oversight, it is more relaxed, allowing for more innovation.

 

LSAs are somewhat of a bridge between the other two categories. These aircraft are not as rigorously certified (using industry consensus standards rather than government mandates), allowing for more innovation, but generally manufacturers build the aircraft with more process control than experimental aircraft.

 

LSA designs are also limited in aircraft weight, number of passengers, and speed, although new rules are being considered to expand these limits and potentially add a new category, light personal aircraft.

 

M&P 02 Experimental Aircraft M&P

 

Composites and processing vary by general aviation category. But broadly speaking, the emphasis is on low cost and moderate performance, with standard uninfiltrated epoxy and E-glass reinforcements being very common (although standard modulus carbon fiber is becoming more common).

 

For the experimental category, the basic wet layup process has historically dominated, whether glass or carbon fiber. Low viscosity, two-part, room temperature curing epoxy is hand mixed in precise weight ratios and then hand laid onto dry cloth, which is then cut to shape and laid onto simple tools or molded foam, which can serve as a flyaway tool for sandwich construction.

 

In recent years, infusion processes such as vacuum assisted resin transfer molding (VARTM) have become increasingly popular. Dry fabrics include 7781 glass fiber and/or 2×2 carbon twill (hybrid layups of mostly glass fiber and a small amount of carbon fiber are common), which are then laid into a mold with a tackifier and bagged; the low viscosity two-part epoxy is pulled into the layup by vacuum.

 

Aircraft like the Arion Lightning or DarkAero’s DarkAero1 are taking advantage of the less messy, better quality parts with greater fiber volume that infusion can provide.

 

Lantor Soric non-fusible core is used for the fiberglass fairing on the rear of the DarkAero 1 canopy.

 

DarkAero’s team of three engineering brothers believed that carbon fiber was the key to achieving its aggressive performance goals (cruising at 275 miles per hour for more than six hours) through improved aerodynamic shape and optimal structural efficiency. DarkAero’s design uses primarily plain weave fabric, with local reinforcement using unidirectional (UD) fabric in areas that carry most or all of the load in one direction.

 

Local stiffness is provided by aramid honeycomb, foam, or a blend of Lantor Soric non-fusible core. For smaller, more complex geometries, such as carbon fiber spinners (the tapered section in front of the propeller), DarkAero uses a 2×2 twill for superior drape and comfort. Parts are room temperature cured on the tool after injection, followed by post-curing simultaneously with bonded subassemblies.

 

DarkAero’s substructure is kept simple and low-cost with large 4-by-8-foot carbon fiber fabric sandwich panels. The final substructure shape is CNC machined, and the subassemblies are bonded together at high temperatures with a paste adhesive. Curing takes place in an oven; autoclaves are too expensive, and out-of-autoclave materials are improving to the point where oven curing is sufficient.

 

DarkAero is going beyond the usual skins and substructures in composite applications. Even the aircraft’s brackets, hardpoints, and bell cranks are made by machining solid billets of infused multiaxial, crimp-free carbon fiber fabric, which enables the company to quickly build quasi-isotropic laminates.

 

Carbon fiber/epoxy-covered aramid honeycomb panels are made from 4-by-8-foot sheets and then CNC-cut to shape to create structurally efficient ribs, shear webs, and bulkheads for the DarkAero 1.

 

DarkAero recognizes that there are many challenges in designing and manufacturing a high-performance single-engine aircraft, but as Keegan Carr, one of the three Carr brothers who founded Dark Aero, says, “Understanding the nuances of composite design and manufacturing is key to the puzzle.”

 

03   M&P 03 Light Sport Aircraft M&P

 

Much like experimental aircraft, LSA composites are often wet-laid or impregnated, but prepregs are increasingly being used to improve quality and performance. Flight Design GmbH (Hoerselberg Hainich, Germany), one of the most popular LSA manufacturers, has moved from wet layup to Hexcel (Stamford, CT, USA) M79 prepreg for its new F2-LSA (the company is also working to certify it as the F2-C23).

 

LSAs are not only found on land and in the air – Icon Aircraft’s (Vacaville, California, U.S.) very sporty A5 is an amphibious product, meaning it can land on a runway or on a body of water such as a lake or bay. Icon decided to use prepreg from the very beginning of the design to gain the maximum performance benefits for its unique land and sea applications. In addition, it saw the manufacturing advantages of prepreg: less labor, faster processing and more consistent results compared to wet layup.

 

The Icon A5 LSA’s Center Spar Layup Of Carbon Fiber Epoxy Prepreg.

 

Icon chose composites because they allow the company to easily create very complex shapes and are corrosion resistant, the latter being a very important consideration for an aircraft that is likely to spend a considerable portion of its life on or around water.

 

More than 95% of the Icon LSA structure is made using 2×2 carbon fiber twill/epoxy prepreg with some localized areas of UD standard modulus carbon fiber prepreg to reinforce high-load (and high directional load) areas.

 

To reduce production costs and improve repeatability, Icon CNC cuts all ply details and uses a laser projector and templates to position the plies in the tool during the manual layup process.

 

In some areas, to increase structural stiffness, rather than adding additional plies, closed-cell foam cores are used to increase the stiffness of the structure as needed, resulting in very small weight penalties and high cost savings.

 

The foam core is CNC machined and has a beveled ramp added to facilitate ply transition. It can then be heat-formed. Composite parts are usually oven cured, but for certain highly loaded structures, such as wing spars, autoclaves are used to reduce porosity and obtain the best possible quality and performance.

 

 

Assembly is primarily paste bonding (epoxy) using sandblasting and solvent wiping for surface preparation. Joint gaps are designed into the components and controlled using a joint fixture. Mixing and application of the paste adhesive is done by hand. Initial cure is at room temperature followed by an elevated post cure.

 

 

Bonded components of the fuselage, center wing box and sides (for stability on the water) of the Icon A5 amphibious aircraft.

 

Despite flying in potentially harsh conditions, the performance of the composites seems to justify the initial design choices. After eight years of service history and several airframes with more than 1,000 flight hours, there have been no failures in bonded joints or laminated components, says Rodolfo Correa, Icon’s engineering director. The company is so pleased with the results that Icon has begun certifying the A5 in addition to continuing to offer an LSA model of the A5.

 

04  M&P 04 Certified Aircraft M&P

 

First generation general aviation certified aircraft (mass production), such as the Diamond/DA-20 or Cirrus SR20, with high composite content, are usually E-glass due to their low cost and ease of inspection (semi-transparent, strong backlighting allows most defects to be discovered by simple visual inspection). However, second generation aircraft such as the Diamond DA-42 or Cirrus SR22 are increasingly using carbon fiber and/or S2 glass, while third generation aircraft such as the DA-62, SF50 Vision Jet and Epic’s E1000 are moving to mostly or all carbon fiber construction to improve their structural efficiency.

 

Similarly, the trend in resin systems is toward higher performance (harder, higher glass transition temperature (Tg)) epoxies that allow for darker paint colors and improved damage resistance. Processing is typically hand layup of prepregs in an oven or autoclave.

 

The Cirrus (Duluth, Minnesota, USA) series SR aircraft is said to be the best-selling single-engine piston general aviation aircraft in recent years and has been an all-composite aircraft since its introduction in the late 1990s.

 

While the company’s latest composites advances have not been disclosed, Cirrus has a long-standing relationship with Toray Advanced Composites (TAC, Tacoma, Washington, U.S.) using its BT250 epoxy system and TC275-1.

 

The latter is a 275°F-cured vacuum bag only (VBO) epoxy prepreg used on the SF50 Vision Jet. Processing is conventional hand-paste assembly.

 

 

Diamond Aircraft designed and built its own in-house on-demand equipment for controlled application of resin to dry fabric to produce “wet prepreg.”

 

One of Cirrus’ main competitors in the all-composite certified general aviation market is Diamond. Leveraging its fiberglass-powered sailplane experience, Diamond still uses “wet prepreg” as a semi-automated way to help control the application of resin in wet layups.

 

Much like Cirrus, Diamond has increased the percentage of carbon fiber in its designs over time from 10% in the early DA20 C1 model (versus 90% glass) to 50% in the DA42, and has fully doubled to 90% carbon fiber versus only about 10% glass in the latest designs, the DA50 RG and DA62.

 

Diamond uses high-strength and standard modulus carbon fiber, which increases material availability (and associated data) and improves airframe performance, driving its increased use.

 

Diamond’s “wet prepreg” process produces in-house prepreg (or wet layups, depending on your perspective) on-demand. The dry fiber rolls are run through custom-designed equipment that meters a certain amount of low-viscosity epoxy to produce a wet prepreg that is immediately cut to shape and placed in the mold.

 

The original resin used in the DA20, DA40, and DA42 was L160 or L285 epoxy from Momentive (Esslingen am Neckar, Germany). Newer designs (DA42-VI, DA50-RG, and DA62) were moved to RIM935 infusion epoxy by Westlake Epoxy because of its high Tg.

 

Early models of Diamond were limited to mostly or all white paint schemes due to the low Tg of L285; moving to RIM935 enabled the company to add new, eye-catching color schemes that are often preferred by customers.

 

Laying Up Carbon Fiber Epoxy “Wet Prepreg” for the right fuselage of the Diamond DA50 RG.

 

Wet prepreg plies can be challenging to work with and a bit messy at times; Diamond has considered introducing automated placement technology to help reduce cycle times and ease the workload for technicians.

 

Diamond has also added resin infusion to its process—using it for parts where porosity is more critical, such as carbon fiber spars (for structural reasons) and fiberglass radomes (lower porosity gives better electromagnetic transmission, and therefore better radar performance).

 

Diamond’s bonded components are surface-finished using peel ply and sandpaper. The bondline resin is the same as the wet prepreg in the structure, but thickened into a paste with cotton pads or microspheres. Mixing and application are all done by hand. The adhesive cures at room temperature, and then the entire structure (laminate and bondlines) is subjected to an elevated temperature post-cure.

 

Diamond DA50 RG fuselage halves and frames being prepared for final assembly bonding. Note that the vertical stabilizer is an integral part of the larger fuselage assembly

 

In Diamond’s 40 years of experience, the company says it has never retired an aircraft due to composite issues. The fuselage is inspected every 6,000 flight hours and typically requires no discovery or repair work. “Composites are in our DNA,” says Robert Kremnzer, head of Diamond Aircraft’s design organization, reflecting on the extensive service life of Diamond’s products. “We wouldn’t think any differently. We wouldn’t design any differently. We think it’s a great material.”

 

Epic Aircraft (Bend, Oregon, U.S.) has transformed a popular kit (experimental) aircraft into one of the highest-performing certified turboprops. Powered by a Pratt & Whitney Canada (Longueuil, QC, Canada) PT6A-67A turboprop engine (its venerable PT6 engine family just recently surpassed 1 billion flight hours), the E1000 and now the E1000 GX can cruise at 380 mph in pressurized comfort conditions, have a range of 2,000 miles, and can reach a maximum altitude of 34,000 feet.

 

 

This Epic E1000 GX fuselage half features Toray 2510 carbon fiber/epoxy prepreg. Note the honeycomb core carriers are coated with purple Henkel EA 9696 epoxy film adhesive.

 

It was performance and durability (e.g., fatigue resistance, which is particularly critical for pressurized fuselages) that first attracted Epic to carbon fiber composites. During the seven-year process of obtaining FAA type and production certification, the company learned and improved the design through testing and retesting, ultimately achieving an extremely strong composite fuselage that was tested to loads approximately twice the highest loads expected during service.

 

In 2021, Epic certified the E1000 GX, now the company’s standard production product. The GX has upgraded avionics and a five-blade composite propeller mounted on the front of the PT6A67A turboprop engine. The new composite propeller improves takeoff performance while reducing noise and increasing passenger comfort.

 

Epic’s Chief Engineer, Brock Strunk, achieved composite certification at Lancair and has extensive experience in industry-wide support for shared composite databases to help general aviation. These efforts include the Advanced General Aviation Technology Experiment, the National Center for Advanced Materials Performance, and CMH-17.

 

Strunk is a major proponent of public databases that allow small aircraft companies to more easily incorporate advanced composites into their designs.

 

 

Epic E1000 GX fuselage halves, bulkheads and firewalls ready for bonding. In the foreground is a carbon fiber/epoxy one-piece (actually wingtip to wingtip) spar, one of two spars designed for flight load redundancy.

 

Epic hand-lays TAC’s carbon fiber and glass fiber 2510 epoxy prepregs to create the approximately 550 composite parts in each aircraft. Close technical relationships with composite and bonding material suppliers are critical, and Epic uses their technical expertise to help optimize the manufacturing process.

 

In addition, by selecting composites that have already been qualified and have a public database, Epic is able to use savings to gain a deeper understanding of how process variability affects final performance, resulting in a robust composite production system.

 

Most of Epic’s parts are made from fabric prepregs with highly loaded structures, such as the wings and horizontal spars using UD prepregs. Local stiffness is typically provided by Hexcel aramid/phenolic honeycomb outer expanded cores and Henkel (Madison Heights, MI, U.S.) Loctite EA 9696 Aero epoxy film adhesive, with the foam core used primarily and to a limited extent in complex geometries that can be thermoformed.

 

Assembly is paste-bonded; the epoxy paste adhesive, also from Henkel, is mixed with an additional thickener to help prevent slumping during fuselage half bonding and wing closure.

 

 

Final structural components of the Epic E1000 GX. The forward fuselage carbon fiber structure is the pressure bulkhead and part of the firewall. The blue area is the surface membrane containing copper lightning protection. Note that the leading edge of the wing is left exposed to allow for the later bonding of the inflatable de-icing boot system

 

05 Potential Future Advances

Technical growth in general aviation composites is likely driven by a single factor: cost. General aviation OEMs are willing to explore new, lightweight options and processing techniques to make airframes more fuel efficient (even electric or hydrogen powered), but switching from proven materials and processes is costly. Not only from the price of raw materials, but also from the time and cost of qualifying and certifying new composite structures.

 

Published databases such as NCAMP and CMH-17 can be a great help to small manufacturers in adopting new materials; material suppliers should consider incorporating a basic set of allowable values ​​into any new structural composite system they bring to market.

 

Driven by market forecasts for composites on general aviation aircraft and advanced air mobility, the composites industry is developing higher performance composites while reducing material, processing and service costs.

 

Advances in toughened epoxies allow them to cure faster at lower temperatures and pressures while still providing autoclave-like performance, enabling the use of stiffer intermediate modulus carbon fibers in UD form. UD fibers are approximately 25-50% more efficient than their woven counterparts. Overall, this results in significant weight savings, taking up less internal wing space, leaving more room for fuel.

 

High-performance semicrystalline thermoplastic resins such as polyetheretherketone (PEEK) and polyetherketoneketone (PEKK) will continue to fall in price and provide better off-aircraft and damage resistance performance than epoxies and even toughened epoxies. High temperature processing requirements present a host of challenges, but new processing and modeling can help address these challenges.

 

Carbon fibers, especially intermediate modulus ones, will find use in primary structures in general aviation due to their unmatched structural properties, especially stiffness. New advances in large tow carbon fibers and new production plants with additional carbon fiber capacity have combined to reduce the cost of carbon fiber, further driving its utilization in general aviation.

 

The development of new composite processes, including advanced fiber placement (AFP) and automated tape laying (ATL), not only allows very high quality and consistent composite structures to be obtained, but also allows the highest structural efficiency from UD carbon fiber, and also allows the manufacture of large, integrated complex structures with reduced assembly costs. Another processing technology, press forming, which uses heated tools and mechanical force for curing, offers the possibility of reducing part processing cycle times from hours or days to minutes.

 

06 A Modest Proposal for Accelerating The Implementation Of Innovation

 

The current revolution in local and regional air transport (as indicated by the term AAM), including vertical takeoff and landing (VTOL), short takeoff and vertical landing (STOVL) and short takeoff and landing (STOL), as well as traditional fixed-wing transport, provides GA aviation with a unique opportunity to use materials and technologies to move their industry forward.

 

Imagine an established GA aviation company partnering with an emerging AAM group (or the collaboration could involve multiple partners in both GA aviation and AAM). They work with material suppliers to jointly determine cost, processing and performance targets for a set of materials (prepregs, adhesives, etc.).

 

The materials are qualified through an FAA-approved data system (such as NCAMP or CMH-17), and the low-cost processing is developed and validated through government/academic composite manufacturing R&D facilities such as the National Institute of Aviation Research (NIAR) Advanced Technology Laboratory for Aerospace Systems (ATLAS) Center in Wichita, Kansas, USA, multi-purpose production facilities or each creates its own factory using knowledge and data.

 

Through the collaboration, the AAM organization gained valuable knowledge from the composite design, analysis and manufacturing expertise that GE Aviation has developed over the years.

 

At the same time, GE Aviation gained new resources to help reduce non-recurring costs, enabling the implementation of new structurally efficient materials and cost-reducing processes.

 

Overall, the future is bright for composites in all areas of general aviation. Leveraging materials and processes developed for other markets will achieve significant performance gains (go farther and faster with less fuel consumption) while improving durability and reducing procurement costs.